Doherty amplifier having envelope control

ABSTRACT

Doherty amplifier having envelope control. In some embodiments, an amplifier system can include a Doherty amplifier having a carrier amplifier and a peaking amplifier, with each of the carrier amplifier and the peaking amplifier including a cascode stage with input and output transistors arranged in a cascode configuration. The amplifier system can further include an envelope tracking bias circuit coupled to the Doherty amplifier and configured to provide a bias signal to the output transistor of the cascode stage of the peaking amplifier.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. Provisional Application Nos.62/585,502 filed Nov. 13, 2017, entitled DOHERTY POWER AMPLIFIER HAVINGENVELOPE CONTROL, and 62/585,503 filed Nov. 13, 2017, entitledQUADRATURE AMPLIFIER HAVING ENVELOPE CONTROL, the disclosure of each ofwhich is hereby expressly incorporated by reference herein in itsentirety.

BACKGROUND Field

The present disclosure relates to envelope control for radio-frequency(RF) amplifiers.

Description of the Related Art

In radio-frequency (RF) applications, envelope tracking technique can beimplemented, where a supply voltage being provided to an amplifier isadjusted based on an envelope of the signal being amplified. Such atechnique can provide, for example, improved efficiency for differentpower associated with the signal.

SUMMARY

According to some implementations, the present disclosure relates to anamplifier system that includes a Doherty amplifier having a carrieramplifier and a peaking amplifier. Each of the carrier amplifier and thepeaking amplifier includes a cascode stage with input and outputtransistors arranged in a cascode configuration. The amplifier systemfurther includes an envelope tracking bias circuit coupled to theDoherty amplifier and configured to provide a bias signal to the outputtransistor of the cascode stage of the peaking amplifier.

In some embodiments, the Doherty amplifier can be a power amplifier. Insome embodiments, the envelope tracking bias circuit can be configuredto provide a bias signal only to the peaking amplifier among the carrierand peaking amplifiers. In some embodiments, the envelope tracking biascircuit can be configured to provide a bias signal only to the outputtransistor of the cascode stage among the transistors of the peakingamplifier.

In some embodiments, each of the input and output transistors of thecarrier amplifier and the peaking amplifier can be implemented as abipolar-junction transistor having a base, an emitter, and a collector.The input transistor of the cascode stage can be implemented in acommon-emitter configuration such that an input signal is received byits base, and its collector is coupled to the emitter of the outputtransistor of the cascode stage. The output transistor of the cascodestage can be implemented in a common-base configuration such that anoutput signal is provided through its collector, and the bias signalfrom the envelope tracking bias circuit is provided to its base.

In some embodiments, each of the input and output transistors of thecarrier amplifier and the peaking amplifier can be implemented as afield-effect transistor having a gate, a source, and a drain. The inputtransistor of the cascode stage can be implemented in a common-sourceconfiguration such that an input signal is received by its gate, and itsdrain is coupled to the source of the output transistor of the cascodestage. The output transistor of the cascode stage can be implemented ina common-gate configuration such that an output signal is providedthrough its drain, and the bias signal from the envelope tracking biascircuit is provided to its gate.

In some embodiments, each of the carrier amplifier and the peakingamplifier can further include a driver stage implemented before therespective cascode stage. The driver stage of each of the carrieramplifier and the peaking amplifier can be configured to operate with aClass AB bias. The input transistor of the cascode stage of each of thecarrier amplifier and the peaking amplifier can be configured to operatewith a Class AB bias. The output transistor of the cascode stage of thecarrier amplifier can be configured to operate with a substantiallyconstant bias voltage.

In some embodiments, the Doherty amplifier can be configured to presenta load current that is less than 10 mA to the envelope tracking biascircuit. In some embodiments, the Doherty amplifier can be configured topresent a load current that is less than 5 mA to the envelope trackingbias circuit. In some embodiments, the load current of the Dohertyamplifier presented to the envelope tracking bias circuit can be lessthan 1/100 of a load current presented to a similar envelope trackingcircuit that provides an envelope tracking supply voltage.

In some embodiments, the Doherty amplifier can be configured to presenta load capacitance that is less than 100 pF to the envelope trackingbias circuit. In some embodiments, the Doherty amplifier can beconfigured to present a load capacitance that is less than 50 pF to theenvelope tracking bias circuit. In some embodiments, the loadcapacitance of the Doherty amplifier presented to the envelope trackingbias circuit can be less than ⅕ of a load capacitance presented to asimilar envelope tracking circuit that provides an envelope trackingsupply voltage.

In some implementations, the present disclosure relates to a Dohertyamplifier that includes a carrier amplifier having a cascode stage withinput and output transistors arranged in a cascode configuration. TheDoherty amplifier further includes a peaking amplifier having a cascodestage with input and output transistors arranged in a cascodeconfiguration. The output transistor of the cascode stage of the peakingamplifier is configured to receive and operate with a bias signal thatis based on an envelope control signal.

According to some teachings, the present disclosure relates to a poweramplifier module that includes a packaging substrate configured toreceive a plurality of components, and a power amplifier systemimplemented on the packaging substrate. The power amplifier systemincludes a Doherty amplifier having a carrier amplifier and a peakingamplifier, with each of the carrier amplifier and the peaking amplifierincluding a cascode stage with input and output transistors arranged ina cascode configuration. The power amplifier system further includes anenvelope tracking bias circuit coupled to the Doherty amplifier andconfigured to provide a bias signal to the output transistor of thecascode stage of the peaking amplifier.

In a number of implementations, the present disclosure relates to awireless device that includes a transceiver configured to generate asignal for transmission, and a power amplifier system configured toamplify the signal. The power amplifier system includes a Dohertyamplifier having a carrier amplifier and a peaking amplifier, with eachof the carrier amplifier and the peaking amplifier including a cascodestage with input and output transistors arranged in a cascodeconfiguration. The power amplifier system further includes an envelopetracking bias circuit coupled to the Doherty amplifier and configured toprovide a bias signal to the output transistor of the cascode stage ofthe peaking amplifier. The wireless device further includes an antennaconfigured to support transmission of the amplified signal.

In accordance with a number of implementations, the present disclosurerelates to an amplifier system that includes a quadrature amplifierhaving first and second amplifiers configured to amplify first andsecond signals in quadrature relative to each other. Each of the firstand second amplifiers includes a cascode stage with input and outputtransistors arranged in a cascode configuration. The amplifier systemfurther includes an envelope tracking bias circuit coupled to thequadrature amplifier and configured to provide a bias signal to theoutput transistor of the cascode stage of at least one of the first andsecond amplifiers.

In some embodiments, each of the first and second amplifiers can be asingle ended amplifier. In some embodiments, each of the first andsecond amplifiers can be a Doherty amplifier. In some embodiments, theDoherty amplifier can be a power amplifier.

In some embodiments, the envelope tracking bias circuit can beconfigured to provide a common bias signal to each of the first andsecond amplifiers. In some embodiments, the envelope tracking biascircuit can be configured to provide a different bias signal to each ofthe first and second amplifiers. In some embodiments, the envelopetracking bias circuit can include a scaling/shifting circuit configuredto generate the different bias signal to each of the first and secondamplifiers. In some embodiments, the envelope tracking bias circuit caninclude a scaling/shifting/delay circuit configured to generate thedifferent bias signal to each of the first and second amplifiers.

In some embodiments, the envelope tracking bias circuit can beconfigured to provide a bias signal only to the output transistor of thecascode stage among the transistors of the first and second amplifiers.

In some embodiments, each of the input and output transistors of thefirst amplifier and the second amplifier can be implemented as abipolar-junction transistor having a base, an emitter, and a collector.The input transistor of the cascode stage can be implemented in acommon-emitter configuration such that an input signal is received byits base, and its collector is coupled to the emitter of the outputtransistor of the cascode stage. The output transistor of the cascodestage can be implemented in a common-base configuration such that anoutput signal is provided through its collector, and the bias signalfrom the envelope tracking bias circuit is provided to its base.

In some embodiments, each of the input and output transistors of thefirst amplifier and the second amplifier can be implemented as afield-effect transistor having a gate, a source, and a drain. The inputtransistor of the cascode stage can be implemented in a common-sourceconfiguration such that an input signal is received by its gate, and itsdrain is coupled to the source of the output transistor of the cascodestage. The output transistor of the cascode stage can be implemented ina common-gate configuration such that an output signal is providedthrough its drain, and the bias signal from the envelope tracking biascircuit is provided to its gate.

In some embodiments, each of the first amplifier and the secondamplifier can further include a driver stage implemented before therespective cascode stage.

In some embodiments, the quadrature amplifier can be configured topresent a load current that is less than 10 mA to the envelope trackingbias circuit. In some embodiments, the quadrature amplifier can beconfigured to present a load current that is less than 5 mA to theenvelope tracking bias circuit. The load current of the quadratureamplifier presented to the envelope tracking bias circuit can be lessthan 1/100 of a load current presented to a similar envelope trackingcircuit that provides an envelope tracking supply voltage.

In some embodiments, the quadrature amplifier can be configured topresent a load capacitance that is less than 100 pF to the envelopetracking bias circuit. In some embodiments, the quadrature amplifier canbe configured to present a load capacitance that is less than 50 pF tothe envelope tracking bias circuit. The load capacitance of thequadrature amplifier presented to the envelope tracking bias circuit canbe less than ⅕ of a load capacitance presented to a similar envelopetracking circuit that provides an envelope tracking supply voltage.

In some implementations, the present disclosure relates to a quadratureamplifier that includes a first amplifier configured to amplify a firstsignal, and having a cascode stage with input and output transistorsarranged in a cascode configuration. The quadrature amplifier furtherincludes a second amplifier configured to amplify a second signal inquadrature with respect to the first signal, and having a cascode stagewith input and output transistors arranged in a cascode configuration.The output transistor of the cascode stage of at least one of the firstand second amplifiers is configured to receive and operate with a biassignal that is based on an envelope control signal.

In a number of teachings, the present disclosure relates to aradio-frequency module that includes a packaging substrate configured toreceive a plurality of components, and an amplifier system implementedon the packaging substrate. The amplifier system includes a quadratureamplifier having first and second amplifiers configured to amplify firstand second signals in quadrature relative to each other, with each ofthe first and second amplifiers including a cascode stage with input andoutput transistors arranged in a cascode configuration. The amplifiersystem further includes an envelope tracking bias circuit coupled to thequadrature amplifier and configured to provide a bias signal to theoutput transistor of the cascode stage of at least one of the first andsecond amplifiers.

In some implementations, the present disclosure relates to a wirelessdevice that includes a transceiver configured to generate a signal fortransmission, and a power amplifier system configured to amplify thesignal. The power amplifier system includes a quadrature amplifierhaving first and second amplifiers configured to amplify first andsecond signals in quadrature relative to each other, with each of thefirst and second amplifiers including a cascode stage with input andoutput transistors arranged in a cascode configuration. The amplifiersystem further includes an envelope tracking bias circuit coupled to thequadrature amplifier and configured to provide a bias signal to theoutput transistor of the cascode stage of at least one of the first andsecond amplifiers. The wireless device further includes an antennaconfigured to support transmission of the amplified signal.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a traditional Doherty power amplifier configured to receivea signal at an input and generate an amplified signal at an output.

FIG. 2 shows an example of a peak in efficiency at a power back-offlevel for the Doherty power amplifier of FIG. 1.

FIG. 3 shows that the efficiency of the traditional Doherty poweramplifier at the efficiency peak of FIG. 2 can be significantly higherthan that of a non-Doherty power amplifier at the same output powerlevel.

FIG. 4 shows a more specific example of the Doherty power amplifier ofFIG. 1.

FIG. 5 shows an example of a change in final stage load impedance as theinput power changes for the Doherty power amplifier of FIG. 4.

FIG. 6 shows an example of a tradeoff between power added efficiency(PAE) and linearity for the Doherty power amplifier of FIG. 4, bysweeping a Class C bias point of the second amplification stage of thepeaking amplifier.

FIG. 7 depicts a block diagram of a Doherty power amplifier systemhaving one or more features as described herein.

FIG. 8 shows a Doherty power amplifier system having a Doherty poweramplifier and an envelope tracking (ET) bias circuit.

FIG. 9 shows a more specific example of the Doherty power amplifiersystem of FIG. 8.

FIG. 10 shows an example of a change in final stage load impedance asthe envelope control voltage changes for the Doherty power amplifiersystem of FIG. 9.

FIG. 11 shows examples of gain and power added efficiency (PAE) fordifferent envelope control voltages provided to the cascode transistorof the peaking amplifier of FIG. 9.

FIG. 12 shows examples of AMAM and AMPM plots for different envelopecontrol voltages provided to the cascode transistor of the peakingamplifier of FIG. 9.

FIG. 13 shows examples of PAE and linearity plots for different envelopecontrol voltages provided to the cascode transistor of the peakingamplifier of FIG. 9.

FIG. 14 shows examples of noise and system efficiency plots for theamplifier of FIG. 9.

FIG. 15 shows measured AMAM waterfall plots for different envelopecontrol voltages provided to the Doherty envelope tracking poweramplifier of FIG. 9.

FIG. 16 shows measured PAE waterfall plots for different envelopecontrol voltages provided to the Doherty envelope tracking poweramplifier of FIG. 9.

FIG. 17 shows the measured PAE waterfall plots of FIG. 16, along with acurve representative of a high PAE for the example envelope controlvoltage range.

FIG. 18 shows plots of envelope control current for different envelopecontrol voltages provided to the cascode transistor of the peakingamplifier of FIG. 9.

FIG. 19 shows a representative load capacitance associated with deliveryof an envelope control voltage to the cascode transistor of the peakingamplifier of FIG. 9.

FIG. 20 shows a cascode-configured amplification stage by itself.

FIG. 21 shows that in some embodiments, a cascode-configuredamplification stage of a power amplifier system can be configured suchthat an envelope control based signal is provided to the cascodetransistor as a bias signal.

FIG. 22 shows a power amplifier system that is similar to the example ofFIG. 21, but with its supply voltage to the collector(s) of thetransistor(s) also being controlled by an envelope tracking circuit.

FIG. 23 depicts a block diagram of a quadrature power amplifier systemthat can include one or more features of the present disclosure.

FIG. 24 depicts an assembly of amplifiers implemented so as to allowamplification of quadrature signals associated with an input signal tobe amplified.

FIG. 25 shows that in some embodiments, each of the amplifiers of FIG.24 can be implemented as a Doherty amplifier.

FIG. 26 shows that in some embodiments, the amplification stages of thepeaking amplifiers of the first and second quadrature amplifiers can beprovided with a common bias signal from a common envelope tracking biascircuit.

FIG. 27 shows a more generic configuration where an amplification stageof a peaking amplifier of a first quadrature amplifier is provided witha first bias signal, and an amplification stage of a peaking amplifierof a second quadrature amplifier is provided with a second bias signal,where the first and second bias signals may or may not be the same.

FIG. 28 shows an example configuration that is similar to the example ofFIG. 26, but in which each of either or both of the bias signal pathscan include a scaling/shifting circuit.

FIG. 29 shows an example configuration that is similar to the example ofFIG. 26, but in which each of either or both of the bias signal pathscan include a scaling/shifting/delay circuit.

FIG. 30 shows that in some embodiments, some or all the wirelesscommunications configurations disclosed herein can be implemented,wholly or partially, in a module.

FIG. 31 illustrates an example wireless device having one or moreadvantageous features described herein.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

A Doherty amplifier can provide significant efficiency advantages over atraditional single ended amplifier. For example, advanced modulationschemes with high peak to average ratios require an amplifier to beoperated several dB from the maximum saturated output power (Psat) tomaintain linearity. Since a Doherty amplifier typically has anefficiency peak 6 dB from Psat, linear efficiency can be improved.However, linearity is often compromised in a Doherty amplifier due to aninherent AMAM/AMPM discontinuity created as the carrier amplifier startscompressing.

Envelope tracking is another technique for achieving high efficiency at,for example, 6 dB power back-off (PBO). Such a technique typicallyrequires an envelope tracking supply modulator, as well as systemcalibration and pre-distortion. For an envelope tracking supplymodulator, performance (e.g., bandwidth/efficiency) is typically afunction of a load current/capacitance presented by the power amplifier.Lower load currents/capacitance typically improve the performance.

Described herein are examples related to Doherty and/or quadratureamplifiers that can be operated with an envelope tracking system, toimprove performance of the amplifier (e.g., high efficiency) as well asan envelope tracking modulator (e.g., reduced load current andcapacitance). While various examples are described herein in the contextof power amplifiers, it will be understood that one or more features ofthe present disclosure can also be implemented with other types ofamplifiers.

FIG. 1 shows a traditional Doherty power amplifier 10 configured toreceive a signal at an input (RF_in) and generate an amplified signal atan output (RF_out). The Doherty power amplifier 10 includes a carrieramplifier 20 and a peaking amplifier 30. In the example shown in FIG. 1,the input signal is split into the carrier amplification path with thecarrier amplifier 20, and into the peaking amplification path with thepeaking amplifier 30. The split signal into the peaking amplifier 30 canbe passed through a quarter-wave transformer 40. Similarly, the outputof the carrier amplifier 20 can be passed through a quarter-wavetransformer 42 before being combined with the output of the peakingamplifier 30. The combined signal can be passed through anotherquarter-wave transformer 44 to provide the amplified signal at theoutput (RF_out).

The example traditional Doherty power amplifier 10 of FIG. 1 combinesthe respective amplified signals from the carrier and peaking amplifiers20, 30 to achieve high efficiency at a power back-off (PBO) level (e.g.,6 dB). For example, FIG. 2 shows a peak in efficiency at the example PBOof 6 dB. Such a peak in efficiency typically occurs at or near theoutput power level when the peaking amplifier is turned on. FIG. 3 showsthat the efficiency of the traditional Doherty power amplifier at theforegoing efficiency peak can be significantly higher than that of anon-Doherty power amplifier at the same output power level (e.g., 6 dBPBO).

FIG. 4 shows a more specific example of the Doherty power amplifier 10of FIG. 1. In FIG. 4, a Doherty power amplifier 10 is shown to include asplitter circuit 12 that splits an input signal received at an input(RFIN) into a carrier amplifier 20 and a peaking amplifier 30. TheDoherty power amplifier 10 is shown to further include a combinercircuit 14 configured to combine the respective amplified signals fromthe carrier amplifier 20 and the peaking amplifier 30.

In the example of FIG. 4, each of the carrier amplifier 20 and thepeaking amplifier 30 is shown to have a two stage configuration, withthe second stage being configured in a cascode configuration. Moreparticularly, the carrier amplifier 20 includes a first amplificationstage 21 and a second amplification stage 22, with the secondamplification stage 22 being configured in a cascode configuration.Similarly, the peaking amplifier 30 includes a first amplification stage31 and a second amplification stage 32, with the second amplificationstage 32 being configured in a cascode configuration.

The carrier amplifier 20 of the Doherty power amplifiers of FIGS. 1 and4 can be configured to operate with, for example, a Class AB bias, andthe peaking amplifier 30 can be configured to operate with, for example,a Class C bias. In some embodiments, load modulation of the carrieramplifier and the power contribution of the peaking amplifier istypically a function of input power. For example, FIG. 5 shows a changein final stage load impedance as the input power changes.

Referring to the example Doherty power amplifier of FIG. 4, the firstamplification stage 21 of the carrier amplifier 20 is shown to beprovided with a Class AB bias; and the input amplifying transistor ofthe cascode-configured second amplification stage 22 of the carrieramplifier 20 is also shown to be provided with a Class AB bias. Thecascode transistor of the cascode-configured second amplification stage22 of the carrier amplifier 20 is shown to be provided with a constantbias voltage at its base/gate, and such a base/gate is shown to becoupled to ground through a capacitance.

Referring to the example Doherty power amplifier of FIG. 4, the firstamplification stage 31 of the peaking amplifier 30 is shown to beprovided with a Class AB bias; and the input amplifying transistor ofthe cascode-configured second amplification stage 32 of the peakingamplifier 30 is also shown to be provided with a Class C bias. Thecascode transistor of the cascode-configured second amplification stage32 of the peaking amplifier 30 is shown to be provided with a constantbias voltage at its base/gate, and such a base/gate is shown to becoupled to ground through a capacitance.

It is also noted that the Doherty power amplifier of FIG. 4 can sufferfrom a tradeoff between power added efficiency (PAE) at 6 dB PBO andlinearity. For example, FIG. 6 shows such a tradeoff by sweeping theClass C bias point of the second amplification stage 32 of the peakingamplifier 30. In FIG. 6, a group of PAE plots 50 shows that as AMAMlinearity is improved (with an increase in peak amplifier bias (arrow51)), the PAE at 6 dB PBO is degraded. Also in FIG. 6, a group of gainplots 52 shows that uniformity of gain at a PBO range (e.g., 5 dB PBO)can change as the peak amplifier bias is changed (arrow 53).

FIG. 7 depicts a block diagram of a Doherty power amplifier system 100that can address some or all of the features and/or challenges describedabove in reference to FIGS. 1-6. In some embodiments, such a Dohertypower amplifier system can includes carrier and peaking amplifiers(collectively indicated as 110), and an envelope tracking (ET) biascircuit 140. Examples related to such a Doherty power amplifier systemare described herein in greater detail.

FIG. 8 shows a Doherty power amplifier system 100 having a Doherty poweramplifier 110 and an envelope tracking (ET) bias circuit 140. Similar tothe example of FIG. 1, the Doherty power amplifier 110 of FIG. 8 can beconfigured to receive a signal at an input (RF_in) and generate anamplified signal at an output (RF_out). The Doherty power amplifier 110includes a carrier amplifier 112 and a peaking amplifier 113. In theexample shown in FIG. 8, the input signal is split into the carrieramplification path with the carrier amplifier 112, and into the peakingamplification path with the peaking amplifier 113. In some embodiments,the split signal into the peaking amplifier 113 can be passed through aquarter-wave transformer 114. Similarly, the output of the carrieramplifier 112 can be passed through a quarter-wave transformer 116before being combined with the output of the peaking amplifier 113. Thecombined signal can be passed through another quarter-wave transformer118 to provide the amplified signal at the output (RF_out).

In some embodiments, a least some portion of the Doherty power amplifierof FIG. 8 can be configured to receive and operate with a bias signalbased on an ET control signal provided by the ET bias circuit 140. Forexample, the carrier amplifier 112 of the Doherty power amplifier 110 ofFIG. 8 can be provided with a Class AB bias, similar to the example ofFIG. 1; and at least a portion of the peaking amplifier 113 of theDoherty power amplifier 110 can be provided with an ET control signal asa bias signal.

FIG. 9 shows a more specific example of the Doherty power amplifiersystem of FIG. 8. In FIG. 9, a Doherty power amplifier system 100 isshown include a Doherty power amplifier 110 configured to receive an ETcontrol signal from an ET bias circuit (140 in FIG. 8) through a nodeENV and a path 142.

In the example of FIG. 9, the Doherty power amplifier 110 is shown toinclude a splitter circuit 116 that splits an input signal received atan input (RFIN) into a carrier amplifier 112 and a peaking amplifier113. The Doherty power amplifier 110 is shown to further include acombiner circuit 118 configured to combine the respective amplifiedsignals from the carrier amplifier 112 and the peaking amplifier 113.

In the example of FIG. 9, each of the carrier amplifier 112 and thepeaking amplifier 113 is shown to have a two stage configuration, withthe second stage being configured in a cascode configuration. Moreparticularly, the carrier amplifier 112 includes a first amplificationstage and a second amplification stage 120, with the secondamplification stage 120 being configured in a cascode configuration.Similarly, the peaking amplifier 113 includes a first amplificationstage and a second amplification stage 130, with the secondamplification stage 130 being configured in a cascode configuration.

In some embodiments, the carrier amplifier 112 of the Doherty poweramplifier 110 of FIG. 8 can be configured to operate with a Class ABbias. For example, the first amplification stage of the carrieramplifier 112 is shown to be provided with a Class AB bias; and theinput amplifying transistor of the cascode-configured secondamplification stage 120 of the carrier amplifier 112 is also shown to beprovided with a Class AB bias. The cascode transistor of thecascode-configured second amplification stage 120 of the carrieramplifier 112 is shown to be provided with a constant bias voltage atits base/gate, and such a base/gate is shown to be coupled to groundthrough a capacitance.

In some embodiments, the first amplification stage of the peakingamplifier 113 can be provided with a Class AB bias; and the inputamplifying transistor of the cascode-configured second amplificationstage 130 of the peaking amplifier 113 can also be provided with a ClassAB bias. In some embodiments, the cascode transistor of thecascode-configured second amplification stage 130 of the peakingamplifier 113 can be configured to be provided with and operate with abias signal based on an ET control signal provided from the ET biascircuit (e.g., 140 in FIG. 8), through the node ENV and the path 142. Inthe example of FIG. 9, such an ET-based bias signal is shown to beprovided to a base/gate of the cascode transistor of thecascode-configured second amplification stage 130 of the peakingamplifier 113. In some embodiments, such a base/gate can be coupled toground through a capacitance.

It will be understood that a Doherty power amplifier having one or morefeatures as described herein can be configured to receive and operatewith an ET-based bias signal at one or more other amplifyingtransistors. It will also be understood that a Doherty power amplifierhaving one or more features as described herein can be implemented withfield-effect transistors (FETs), bipolar-junction transistors (BJTs), orsome combination thereof.

In some embodiments, a Doherty power amplifier (e.g., 110 in FIGS. 8 and9) having one or more features as described herein can be configured tocombine signals from carrier and peaking amplifiers to achieve highefficiency at, for example, 6 dB PBO. It is noted that load modulationof the carrier amplifier (112 in FIG. 9) and power contribution of thepeaking amplifier (113 in FIG. 9) is now a function of the ET controlsignal (e.g., envelope control voltage). For example FIG. 10 shows achange in final stage load impedance as the envelope control voltagechanges.

FIGS. 11-18 show examples of performance characteristics of the exampleDoherty power amplifier 110 of FIG. 9.

In FIG. 11, gain (left panel) and PAE (right panel) are shown fordifferent envelope control voltages provided to the cascode transistorof the peaking amplifier 113 of FIG. 9. It is noted that such plots aregenerally consistent with envelope tracking power amplifiers. Referringto the gain plot of FIG. 11, it is further noted that 7 dB of usablehigh PAE dynamic range is provided in the Doherty power amplifier 110 ofFIG. 9.

In FIG. 12, AMAM (left panel) and AMPM (right panel) plots are shown fordifferent envelope control voltages provided to the cascode transistorof the peaking amplifier 113 of FIG. 9. Also shown are calibrated AMAMplot 150 and calibrated AMPM plot 151 based on the operation of thepower amplifier 110 with an envelope control based bias signal.

In FIG. 13, PAE (left panel) and linearity (right panel) plots areshown. More particularly, the PAE plots on the left panel are fordifferent envelope control voltages provided to the cascode transistorof the peaking amplifier 113 of FIG. 9, and a PAE curve 152 represents acalibrated PAE contour with an envelope control based bias signal.

Referring to the linearity plots on the right panel of FIG. 13, curves153 (LTE ACLR1) and 155 (LTE ACLR2) are for a non-Doherty envelopetracking power amplifier operating at 8V, and curves 154 (LTE ACLR1) and156 (LTE ACLR2) are for the Doherty envelope tracking power amplifier ofFIG. 9 operating at 11V.

In FIG. 14, noise (left panel) and system efficiency (right panel) plotsare shown. More particularly, plot 157 of the left panel is for a noisein a receive (Rx) band resulting from operation of a non-Dohertyenvelope tracking power amplifier, and plot 158 is for a noise in thereceive (Rx) band resulting from operation of the Doherty envelopetracking power amplifier of FIG. 9.

Referring to FIG. 14, plot 160 in the right panel is for an efficiencyof power amplifier-only for a non-Doherty envelope tracking poweramplifier, and plot 162 is for an efficiency of power amplifier-only forthe Doherty envelope tracking power amplifier of FIG. 9. Further, plot159 in the right panel is for an efficiency of the system associatedwith the non-Doherty envelope tracking power amplifier, and plot 161 isfor an efficiency of the system (100) associated with the Dohertyenvelope tracking power amplifier of FIG. 9. It is noted that the systemefficiency (45%) associated with the Doherty envelope tracking poweramplifier of FIG. 9 is significantly higher than that of the non-Dohertyenvelope tracking power amplifier.

In FIG. 15, measured AMAM waterfall plots are shown for differentenvelope control voltages provided to the Doherty envelope trackingpower amplifier of FIG. 9. In FIG. 16, measured PAE waterfall plots areshown for different envelope control voltages provided to the Dohertyenvelope tracking power amplifier of FIG. 9. In FIG. 17, measured PAEwaterfall plots of FIG. 16 are shown, along with a curve 163representative of a high PAE for the example envelope control voltagerange. For such a high PAE curve, example PAE values are shown fordifferent PBO levels.

In some embodiments, a Doherty power amplifier system having one or morefeatures as described herein can be configured to be significantly moreefficient than an envelope tracking system associated with a non-Dohertypower amplifier. For example, FIG. 14 shows that the Doherty poweramplifier system of FIGS. 8 and 9 has a significantly higher systemefficiency than that of a non-Doherty power amplifier system. Such animprovement in system efficiency can be due to, for example, reducedenvelope control current, reduced load capacitance associated with anenvelope control signal, or some combination thereof.

FIG. 18 shows plots of envelope control current for different envelopecontrol voltages provided to the cascode transistor of the peakingamplifier 113 of FIG. 9. One can see that for essentially all of theoutput power range, such envelope control current values are less than 5mA. For comparison, typical load current values associated with anon-Doherty envelope tracking power amplifier system are in a range ofabout 500 mA to 1,000 mA.

FIG. 19 shows a representative load capacitance 143 associated withdelivery of the envelope control voltage (through the path 142) to thecascode transistor of the peaking amplifier 113 of FIG. 9. In FIG. 19,such a load capacitance is shown to have a value of about 40 pF.Accordingly, one can expect a load capacitance that is less than 50 pFfor a Doherty power amplifier system having one or more features asdescribed herein. For comparison, typical load capacitance valuesassociated with a non-Doherty envelope tracking power amplifier systemare in a range of about 250 pF to 500 pF.

As described herein, an envelope control signal can be provided to acascode-configured amplification stage of a Doherty power amplifier. Inthe example of FIG. 9, such a cascode-configured amplification stage isa second stage (130) that follows a first stage. It will be understoodthat a Doherty power amplifier having one or more features as describedherein may or may not have such a first stage. Accordingly, acascode-configured amplification stage that receives and operates withan envelope control signal can be the only amplification stage, or beone of a plurality of amplification stages, of a Doherty poweramplifier.

FIG. 20 shows a cascode-configured amplification stage 130 by itself. Inthe example context of BJTs, such an amplification stage can include afirst transistor Q1 configured to receive an input signal (IN) throughits base and output a partially amplified signal through its collector,with its emitter typically being grounded. The collector of Q1 is shownto be coupled to an emitter of a second transistor (also referred to asa cascode transistor); thus, the partially amplified signal from Q1 isreceived by Q2 through its emitter, and Q2 further amplifies and outputsthe resulting amplified signal through its collector to an output nodeOUT.

In the example of FIG. 20, Q1 is shown to be biased with a bias signal(e.g., Class AB bias) through its base, and Q2 is shown to be biasedwith a bias signal that is based on an envelope control signal asdescribed herein. Such an envelope control based bias signal is shown tobe provided to the base of Q2 through an envelope control node(Envelope) and the path 142.

In some embodiments, the envelope control signal, and thus thecorresponding bias signal for Q2, can be configured for amplificationoperations involving LTE cellular/data signals. It will be understoodthat one or more features of the present disclosure can also beimplemented to support other wireless signals.

It will also be understood that one or more features of the presentdisclosure can also be implemented for a cascode-configuredamplification stage having other types of amplifying transistors. Forexample, the foregoing description in reference to FIG. 20 can alsoapply to amplifying transistors implemented as field-effect transistors(FETs).

FIG. 21 shows that in some embodiments, a cascode-configuredamplification stage of a power amplifier system 100 (e.g., a Dohertypower amplifier system) having one or more features as described herein,including a configuration associated with a Doherty power amplifier, canbe configured such that an envelope control based signal is provided tothe cascode transistor (e.g., to the base of the cascode transistor) asa bias signal. In such an example configuration, a supply voltage to thecascode transistor (e.g., to the collector of the cascode transistor)can be provided by a separate source.

More particularly, in FIG. 21, one or more cascode circuits (n circuits,with n being a positive integer greater than or equal to 1) can bearranged in parallel to form a cascode-configured amplification stage130. In such a configuration, the first transistor of each cascodecircuit (e.g., common-emitter transistor M11, . . . , or M1 n) is shownto receive an input signal through its base, distributed from the input(IN) and generate a partially amplified signal through its collector.The collector of each M1 is shown to be coupled to the emitter of thecorresponding second transistor (cascode transistor, Q21, . . . , or Q2n), such that the partially amplified signal from M1 is provided to theemitter of the corresponding cascode transistor Q2, so as to generate afurther amplified signal through Q2's collector. If there are more thanone cascode circuits, the outputs of the collectors of the cascodetransistors Q2 s can be combined to provide an amplified signal at theoutput node (OUT).

In the example of FIG. 21, the base(s) of the M1 transistor(s) can beprovided with a common bias signal, separate bias signals, or somecombination thereof. Further, the base(s) of the Q2 transistor(s) can beprovided with an envelope control based bias signal as described herein.In the Example of FIG. 21, such an envelope control based bias signalcan be provided by an envelope tracking bias circuit 120 that includesan envelope tracker circuit and a dynamic bias circuit 170 implementedbetween the envelope tracker and the base(s) of the Q2 transistor(s). Inthe example shown, the dynamic bias circuit 170 can include a parallelcombination of a resistance Rb1 and a capacitance Cb1 implementedbetween the output of the envelope tracker and the base(s) of the Q2transistor(s). The dynamic bias circuit 170 can further include aparallel combination of resistance Rb2 and a capacitance Cb2 implementedbetween the base(s) of the Q2 transistor(s) and ground. In someembodiments, some or all of the resistances and capacitances can beconfigured to have fixed values, adjustable values, or any combinationthereof.

In the example of FIG. 21, the envelope control based bias signal isshown to be provided to the base(s) of the Q2 transistor(s), but not tothe collector(s). In the example of FIG. 21, the collector(s) of the Q2transistor(s) is/are shown to be provided with a supply voltage througha supply circuit 172 implemented between an input voltage node Vin and asupply node Vdc_trck.

FIG. 22 shows a power amplifier system 100 that is similar to theexample of FIG. 21, but with its supply voltage to the collector(s) ofthe Q2 transistor(s) also being controlled by an envelope trackingcircuit 120. Thus, in the example of FIG. 22, the envelope trackingcircuit 120 can be configured to provide the envelope control based biassignal to the base(s) of the Q2 transistor(s) similar to the example ofFIG. 21. The envelope tracking circuit 120 can be further configured tomodulate the supply voltage of the supply circuit 172 based on theoutput of the envelope tracker.

Various examples described herein in reference to FIGS. 1-22 generallyrelate to Doherty power amplifiers. FIGS. 23-29 show that in someembodiments, one or more features of the present disclosure can also beimplemented in other types of power amplifier configurations. Forexample, FIG. 23 depicts a block diagram of a quadrature power amplifiersystem 200 that can include one or more features of the presentdisclosure. In some embodiments, such a quadrature power amplifiersystem 200 can include amplifiers configured and operated in quadrature(indicated as 210), and an envelope tracking bias circuit 220.

FIG. 24 depicts an assembly 210 of amplifiers 212, 213 (e.g., poweramplifiers) implemented so as to allow amplification of quadraturesignals (e.g., I and Q signals) associated with an input signal (RF_in)to be amplified. Such quadrature signals can be generated from the inputsignal by an input circuit 202, and the amplified quadrature signalsfrom the amplifiers 212, 213 can be combined by a combiner circuit 204to generate an output signal (RF_out).

FIG. 24 further shows that in some embodiments, at least some of theamplifiers 212, 213 can be provided with a bias signal that is based onan envelope control signal. In FIG. 24, such a bias signal can beprovided by an envelope tracking bias circuit 220.

In some embodiments, the envelope tracking bias circuit 220 can besimilar to the examples described herein in reference to the Dohertypower amplifier system. In some embodiments, the envelope tracking biascircuit 220 can be different from the Doherty power amplifier basedexamples.

In some embodiments, each of the amplifiers 212, 213 can be implementedas a single ended amplifier with one or more amplification stages, or asa Doherty amplifier. For example, FIG. 25 shows a configuration whereeach of the amplifiers 212, 213 is implemented as a Doherty amplifier.Accordingly, the two Doherty amplifiers 212, 213 are parts of anassembly 210 of amplifiers configured to amplify quadrature signals.

In the example of FIG. 25, each of the two quadrature signal paths isshown to include a pre-amplifier (214 or 215). It will be understoodthat in some embodiments, such a pre-amplifier may or may not beincluded in the assembly of quadrature amplifiers 210.

In the example of FIG. 25, each of the carrier amplifier and the peakingamplifier of each of the two Doherty amplifiers 212, 213 is shown toinclude two stages, similar to the example of FIG. 9. It will beunderstood that in some embodiments, such a carrier amplifier and/or thepeaking amplifier can include a single stage (e.g., a cascode-configuredstage or otherwise), or more than two stages.

In the example of FIG. 25, the quadrature amplifier system 200 canfurther include an envelope tracking bias circuit 220 configured toprovide a bias signal to at least some of the quadrature amplifiersystem 200 based on an envelope control signal. In some embodiments,such an envelope tracking bias circuit can be configured to be similarto the various examples described herein in reference to FIGS. 7-22.

FIGS. 26-29 show various examples of a quadrature amplifier system 200having quadrature amplifiers 212, 213. Each of such quadratureamplifiers is shown to be implemented as a Doherty amplifier. Moreparticularly, the first quadrature amplifier 212 is shown to include acarrier amplifier having a first stage and a second stage 220 a, and apeaking amplifier having a first stage and a second stage 230 a.Similarly, the second quadrature amplifier 213 is shown to include acarrier amplifier having a first stage and a second stage 220 b, and apeaking amplifier having a first stage and a second stage 230 b. In someembodiments, each of the second stages 220 a, 230 a, 220 b, 230 b of thefirst and second quadrature amplifiers 212, 213 can be implemented as acascode-configured stage, similar to the example of FIG. 9.

In some embodiments, each peaking amplifier of the two quadratureamplifiers 212, 213 can be provided with and operate with a bias signalthat is based on an envelope control signal. For example, in the contextof the peaking amplifier having the cascode-configured stage (e.g., 130in FIG. 9), the cascode transistor of such a stage can be provided withand operate with such a bias signal, similar to the example of FIG. 9.

FIG. 26 shows that in some embodiments, the amplification stages 230 a,230 b of the peaking amplifiers of the first and second quadratureamplifiers 212, 213 can be provided with a common bias signal from acommon envelope tracking bias circuit 220. In FIG. 26, such an envelopetracking bias circuit is shown to provide a common envelope controlsignal to the amplification stage 230 a through a path 242 a, and to theamplification stage 230 b through a path 242 b.

FIG. 27 shows a more generic configuration where the amplification stage230 a of the peaking amplifier of the first quadrature amplifier 212 isprovided with a bias signal (through a path 242 a) from an envelopetracking bias circuit 220 a, and the amplification stage 230 b of thepeaking amplifier of the second quadrature amplifier 213 is providedwith a bias signal (through a path 242 b) from an envelope tracking biascircuit 220 b. In FIG. 27, such bias signals may or may not be the samebias signal, and the envelope tracking bias circuits 220 a, 220 b may beparts of the same envelope tracking bias circuit, or may be differentenvelope tracking bias circuits.

In some embodiments, the envelope control based bias signals provided tothe stages 230 a, 230 b can be different. For example, one bias signalcan be adjusted in amplitude and/or be offset relative to the other biassignal to reduce mismatch and therefore improve performance.

FIG. 28 shows an example configuration that is similar to the example ofFIG. 26 (in the context of a common envelope tracking bias circuit 220),but in which each of either or both of the bias signal paths 242 a, 242b can include a scaling/shifting circuit (244 a or 244 b). In such aconfiguration, a bias signal provided to the corresponding stage (230 aor 230 b) can be linearly derived from an envelope control signal (ENVCTRL) by shifting and rescaling. Accordingly, the resulting bias signalsmay be the same, or be different from each other. In some embodiments,linear coefficients can be programmed, derived from a mismatchmeasurement, or any combination thereof.

FIG. 29 shows an example configuration that is similar to the example ofFIG. 26 (in the context of a common envelope tracking bias circuit 220),but in which each of either or both of the bias signal paths 242 a, 242b can include a scaling/shifting/delay circuit (246 a or 246 b). In sucha configuration, a bias signal provided to the corresponding stage (230a or 230 b) can be linearly derived from an envelope control signal (ENVCTRL) by shifting and rescaling similar to the example of FIG. 28, andcan also include a delay. Accordingly, the resulting bias signals may bethe same, or be different from each other. In some embodiments, variousparameters associated with such adjustments can allow fine adjustmentsof timing on either or both branches 242 a, 242 b. Some or all of suchparameters can be factory set, programmed based on center frequency orother known parameters of RF waveforms, etc.

FIG. 30 shows that in some embodiments, some or all the wirelesscommunications configurations disclosed herein can be implemented,wholly or partially, in a module. In the example of FIG. 30, a module1300 can include a packaging substrate 1301, and a number of componentscan be mounted on such a packaging substrate 1301. For example, a frontend module power module integrated circuit (FEM-PMIC) component 1332, apower amplifier system (100 or 200) which includes an envelope tracker1303, a transceiver 1320, and a multiplexer assembly 1306 can be mountedand/or implemented on and/or within the packaging substrate 1301. Othercomponents such as a number of SMT devices 1334 and an antenna switchmodule (ASM) 1336 can also be mounted on the packaging substrate 1301.Although all of the various components are depicted as being laid out onthe packaging substrate 1301, it will be understood that somecomponent(s) can be implemented over other component(s).

In some implementations, a device and/or a circuit having one or morefeatures described herein can be included in an RF electronic devicesuch as a wireless device. Such a device and/or a circuit can beimplemented directly in the wireless device, in a modular form asdescribed herein, or in some combination thereof. In some embodiments,such a wireless device can include, for example, a cellular phone, asmart-phone, a hand-held wireless device with or without phonefunctionality, a wireless tablet, etc.

FIG. 31 illustrates an example wireless device 1400 having one or moreadvantageous features described herein. In the context of a modulehaving one or more features as described herein, such a module can begenerally depicted by a dashed box 1421, and can be implemented as, forexample, a front-end module (FEM) and a diversity receiver (DRx) module1451 (which can be implemented within a diversity receiver system)coupled to a diversity antenna 1440 and a transceiver 1420.

The wireless device 1400 can include power amplifiers (PAs) (110 or 210)that receive their respective RF signals from the transceiver 1420 thatcan be configured and operated in known manners to generate RF signalsto be amplified and transmitted, and to process received signals. Suchpower amplifiers, along with a respective envelope tracker (140 or 220)can be parts of a respective power amplifier system (100 or 200) asdescribed herein. The transceiver 1420 is shown to interact with abaseband sub-system 1405 that is configured to provide conversionbetween data and/or voice signals suitable for a user and RF signalssuitable for the transceiver 1420. The transceiver 1420 can also be incommunication with a power management component 1407 that is configuredto manage power for the operation of the wireless device 1400. Suchpower management can also control operations of the baseband sub-system1405 and the modules 1421, 1451.

The baseband sub-system 1405 is shown to be connected to a userinterface 1401 to facilitate various input and output of voice and/ordata provided to and received from the user. The baseband sub-system1405 can also be connected to a memory 1409 that is configured to storedata and/or instructions to facilitate the operation of the wirelessdevice, and/or to provide storage of information for the user.

In the example wireless device 1400, outputs of the PAs (110 or 210) areshown to be matched (via respective output match circuits 1426) androuted to their respective diplexers 1427. Such amplified and filteredsignals can be routed to an antenna 1430 (or multiple antennas) throughan antenna switch 1428 for transmission. In some embodiments, thediplexers 1427 can allow transmit and receive operations to be performedsimultaneously using a common antenna (e.g., 1430). Received signals areshown to be routed to “Rx” paths (not shown) that can include, forexample, a low-noise amplifier (LNA). In some implementations, thereceive paths also include one or more filters (not shown). The frontend module 1421 can include a multiplexer 1429 configured to directsignals from the transceiver 1429 to the PAs (110 or 210).

The wireless device 1400 includes an envelope tracker (140 or 210)configured to generate envelope tracking signals to bias and/or powerthe power amplifiers (110 or 210) as described herein.

The wireless device also includes a diversity antenna 1440 and adiversity receiver module 1451 that receives signals from the diversityantenna 1440. The diversity receiver module 1451 transmits receivedsignals to the transceiver 1410. In some embodiments, a diplexer ortriplexer can be included between the diversity antenna 1440 and thediversity receiver module 1451 and/or between the primary antenna 1430and the front end module 1421.

A number of other wireless device configurations can utilize one or morefeatures described herein. For example, a wireless device does not needto be a multi-band device. In another example, a wireless device caninclude additional antennas such as diversity antenna, and additionalconnectivity features such as Wi-Fi, Bluetooth, and GPS.

Additional examples related to Doherty power amplifiers can be found in,for example, U.S. Pat. No. 9,806,681, titled DOHERTY POWER AMPLIFIERHAVING AM-AM COMPENSATION, which is expressly incorporated by referencein its entirely.

Additional examples related to envelope-based biasing of a poweramplifier can be found in, for example, U.S. patent application Ser. No.15/596,149, titled POWER AMPLIFICATION SYSTEM WITH ENVELOPE-BASED BIAS,which is expressly incorporated by reference in its entirely.

Additional examples related to envelope tracking of power amplifiers canbe found in, for example, U.S. patent application Ser. No. 15/811,394,titled ENVELOPE TRACKING FOR HIGH POWER AMPLIFIERS, which is expresslyincorporated by reference in its entirely.

Additional examples related to power amplification system withpre-amplifier (e.g., 214 and 215 in FIG. 25 of the present disclosure)can be found in, for example, U.S. Publication No. 2016/0241295, titledPOWER AMPLIFICATION SYSTEM WITH COMMON BASE PRE-AMPLIFIER, which isexpressly incorporated by reference in its entirely.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Additionally, the words “herein,” “above,” “below,” and words of similarimport, when used in this application, shall refer to this applicationas a whole and not to any particular portions of this application. Wherethe context permits, words in the above Detailed Description using thesingular or plural number may also include the plural or singular numberrespectively. The word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list, and anycombination of the items in the list.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While some embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. An amplifier system comprising: a Dohertyamplifier including a carrier amplifier and a peaking amplifier, each ofthe carrier amplifier and the peaking amplifier including a cascodestage with input and output transistors arranged in a cascodeconfiguration; an envelope tracking bias circuit coupled to the Dohertyamplifier and configured to provide a bias signal to the outputtransistor of the cascode stage of the peaking amplifier; and a supplycircuit configured to provide a non-envelope tracking supply voltage tothe output transistor of the cascode stage of the peaking amplifier. 2.The amplifier system of claim 1 wherein the Doherty amplifier is a poweramplifier.
 3. The amplifier system of claim 1 wherein the envelopetracking bias circuit is configured to provide a bias signal only to thepeaking amplifier among the carrier and peaking amplifiers.
 4. Theamplifier system of claim 1 wherein the envelope tracking bias circuitis configured to provide a bias signal only to the output transistor ofthe cascode stage among the transistors of the peaking amplifier.
 5. Theamplifier system of claim 1 wherein each of the input and outputtransistors of the carrier amplifier and the peaking amplifier isimplemented as a bipolar-junction transistor having a base, an emitter,and a collector.
 6. The amplifier system of claim 5 wherein the inputtransistor of the cascode stage is implemented in a common-emitterconfiguration such that an input signal is received by its base, and itscollector is coupled to the emitter of the output transistor of thecascode stage.
 7. The amplifier system of claim 6 wherein the outputtransistor of the cascode stage is implemented in a common-baseconfiguration such that an output signal is provided through itscollector, and the bias signal from the envelope tracking bias circuitis provided to its base.
 8. The amplifier system of claim 1 wherein eachof the input and output transistors of the carrier amplifier and thepeaking amplifier is implemented as a field-effect transistor having agate, a source, and a drain.
 9. The amplifier system of claim 8 whereinthe input transistor of the cascode stage is implemented in acommon-source configuration such that an input signal is received by itsgate, and its drain is coupled to the source of the output transistor ofthe cascode stage.
 10. The amplifier system of claim 9 wherein theoutput transistor of the cascode stage is implemented in a common-gateconfiguration such that an output signal is provided through its drain,and the bias signal from the envelope tracking bias circuit is providedto its gate.
 11. The amplifier system of claim 1 wherein each of thecarrier amplifier and the peaking amplifier further includes a driverstage implemented before the respective cascode stage.
 12. The amplifiersystem of claim 11 wherein the driver stage of each of the carrieramplifier and the peaking amplifier is configured to operate with aClass AB bias.
 13. The amplifier system of claim 11 wherein the inputtransistor of the cascode stage of each of the carrier amplifier and thepeaking amplifier is configured to operate with a Class AB bias.
 14. Theamplifier system of claim 13 wherein the output transistor of thecascode stage of the carrier amplifier is configured to operate with asubstantially constant bias voltage.
 15. The amplifier system of claim 1wherein the Doherty amplifier presents a load current that is less than10 mA to the envelope tracking bias circuit.
 16. The amplifier system ofclaim 15 wherein the Doherty amplifier presents a load current that isless than 5 mA to the envelope tracking bias circuit.
 17. The amplifiersystem of claim 16 wherein the load current of the Doherty amplifierpresented to the envelope tracking bias circuit is less than 1/100 of aload current presented to a similar envelope tracking circuit thatprovides an envelope tracking supply voltage.
 18. The amplifier systemof claim 1 wherein the Doherty amplifier presents a load capacitancethat is less than 100 pF to the envelope tracking bias circuit.
 19. ADoherty amplifier comprising: a carrier amplifier having a cascode stagewith input and output transistors arranged in a cascode configuration;and a peaking amplifier having a cascode stage with input and outputtransistors arranged in a cascode configuration, the output transistorof the cascode stage of the peaking amplifier configured to receive andoperate with a bias signal that is based on an envelope control signal,the output transistor of the cascode stage of the peaking amplifierfurther configured to receive and operate with a non-envelope trackingsupply voltage.
 20. A power amplifier module comprising: a packagingsubstrate configured to receive a plurality of components; and a poweramplifier system implemented on the packaging substrate, and including aDoherty amplifier having a carrier amplifier and a peaking amplifier,each of the carrier amplifier and the peaking amplifier including acascode stage with input and output transistors arranged in a cascodeconfiguration, the power amplifier system further including an envelopetracking bias circuit coupled to the Doherty amplifier and configured toprovide a bias signal to the output transistor of the cascode stage ofthe peaking amplifier, the power amplifier system further including asupply circuit configured to provide a non-envelope tracking supplyvoltage to the output transistor of the cascode stage of the peakingamplifier.